Method for synchronization in a local area network including a store-and-forward device

• US 8,291,105 B2
• Filed: 12/28/2001
• Issued: 10/16/2012
• Est. Priority Date: 12/28/2000
• Status: Expired due to Fees

• Current Assignee: ABB Research Ltd. (ABB Ltd.)
• Original Assignee: ABB Research Ltd. (ABB Ltd.)
• Inventors: Holmeide, Øyvind, Apneseth, Christoffer, Johannessen, Svein, Lokstad, Trond, Skeie, Tor
• Primary Examiner(s): Neurauter, George C
• Application Number:
  US10/465,945
• Publication Number:
  US 20040151125A1
• Time in Patent Office: 3,945 Days
• Field of Search: 709/248, 709/232, 370/252
• US Class Current: 709/232
• CPC Class Codes: H04J 3/0667 Bidirectional timestamps, e.g. NTP or PTP for compensation of clock drift and for compensation of propagation delays arrangements for monitoring round trip delays in packet switching networks H04L43/0864
• CPC Class Codes: H04L 43/50 Testing arrangements

Abstract

The present invention relates to a method for identifying inaccurate time measurements in a local area network. The local area network comprises a sending device (2), such as a time client, a destination device (3), such as a timeserver, and a switching device and/or a store-and-forward device (6). A time request packet (4) is rejected if it arrives at the time client or timeserver within a given or calculated minimum distance from the preceding network packet. The time request packet (4) is also rejected if it falls outside a set of calculated error limits based on a set of estimates. Those estimates are kept in the time client memory and updated according to a given set of rules.

Claims

1. A method for identifying inaccurate time measurements when running time synchronization on a local area network, the local area network comprising a sending device, a destination device and a switching device and/or a store-and-forward device arranged on a path between the sending device and the destination device, the method comprising:

storing a value for an estimated relative local frequency deviation (dfR) and a confidence comparison value for a local time-of-day clock in a memory in the sending device;

obtaining a time value (tP) of the local time-of-day clock at a previous time adjustment;

obtaining an estimated local frequency accuracy (d2fR) at the previous time adjustment;

obtaining an estimated local time precision (dtP) at the previous time adjustment;

receiving time stamps;

calculating a new time adjustment (tA) from the received time stamps;

obtaining a time value (tR) of the local time-of-day clock for the new time adjustment (tA);

calculating an upper acceptance level (tH) and a lower acceptance level (tL) for the new time adjustment (tA) based on the following equations;

tH=tB+dtB

tL=tB−

dtB

where tB=tP+(tR−

tP)*(1+dfR) and

dtB=dtP+(tR−

tP)*d2fR; and

determining whether the new time adjustment (tA) falls between tH and tL.

2. The method of claim 1, wherein the sending device is a time client.

3. The method of claim 1, further comprising:

if the new time adjustment (tA) falls between tH and tL, accepting the new time adjustment (tA) and updating the time value of the local time-of-day clock.

4. A method for identifying inaccurate time measurements when running time synchronization on a local area network, the local area network comprising a sending device, a destination device and a switching device and/or a store-and-forward device connected between the sending device and the destination device, the method comprising:

receiving a network packet and then a time request packet at the destination device;

measuring the time between the received network packet and the received time request packet; and

rejecting the received time request packet if the measured time is within a predetermined time interval, wherein the predetermined time interval is calculated using a known length of a time request packet and a required inter-packet gap.

5. The method of claim 4, wherein the step of calculating the predetermined time interval comprises calculating a minimum time interval using the known length of a time request and the required inter-packet gap and adding a tolerance to the minimum time interval.